This is the case in many places outside of the US as well, as far as I can tell. The included extension cord (C5?) was grounded at least for my last MacBook years ago, but they provide a non-grounded connector probably because it's less bulky (easier to design as a folding mechanism) and more universal (especially in continental Europe where the two-prong is nearly universal and the third prong is far from that).

It's also not just Apple. Lower wattage Lenovo chargers (including ThinkPad) have two-prong adapters and plugs.